Three major explosive eruptions occured at Kasatochi between approximately 2200 UTC (14:00 AKDT) on 07 August and 0435 UTC on 08 August (20:35 AKDT on 07 August). Ash from these explosions reached at least 45,000 ft above sea level. Ash emissions became continuous following the last explosive event and have produced a continuous ash cloud extending for more than 500 miles in a counterclockwise spiral. Ash from this continuous phase is likely at an altitude in excess of 35,000 ft above sea level. The eruption is ongoing and additional explosive events are possible.

Kasatochi Island and the general area around the island are within a hazard zone and should be avoided. Significant ash fall, pyroclastic flows and surges, and ballistic fallout may occur without warning. If the eruption intensifies and large pyroclastic flows and surges are generated, these may travel over water for tens of miles. It is also possible for these flows to initiate waves if they interact dynamically with the sea.

Kasatochi Island represents the emergent summit of a predominantly submarine volcano composed of basaltic and andesitic flows and pyroclastics. Kasatochi has had no known historical eruptions; however, the volcano is mantled by a cover of young appearing pyroclastic debris indicative of relatively recent explosive activity. The island consists of a single, undissected cone with a central lake-filled crater about 0.75 km in diameter. A maximum height of 314 m is on the southern crater rim; elevation of the lake is less than about 60 m. Kasatochi Island is at the northern end of a 15-km-long, 6-km-wide submarine ridge that is normal to the trend of the Andreanof Islands. Water depths along the ridge are less than 90 m; if Kasatochi is constructed entirely on the ridge, the total height of the volcanic pile is only a little more than 400 m. Kasatochi is 83 km (52 mi) east of the community of Adak , and 90 km (55 mi) west of the community of Atka.AVO is monitoring this situation closely and has its operations room staffed 24 hours per day. More information about activity at Kasatochi Volcano can be found on the AVO web site at www.avo.alaska.edu or by calling 907-786-7497.

Volcanic unrest at Cleveland Volcano has declined in intensity. A weak thermal anomaly has been observed intermittently at the summit over the past week when not obscured by clouds and drifting ash from the eruption of Kasatochi. AVO has received no new information about activity at Cleveland Volcano for several days. The weak thermal anomaly at the summit is likely the result of the cooling of the newly emplaced lava flows. No ash plumes have been observed in satellite images since July 29.

Cleveland volcano forms the western half of Chuginadak Island, a remote and uninhabited island in the east central Aleutians. It is located about about 75 km (45 mi.) west of the community of Nikolski, and 1500 km (940 mi.) southwest of Anchorage. The volcano's most recent significant eruption began in February, 2001 and had 3 explosive events that produced ash clouds as high as 12 km (39,000 ft) above sea level. This eruption also produced a rubbly lava flow and hot avalanche that reached the sea. The most recent minor ash emissions were observed in October 2006.

Eruptive activity continues at Okmok Volcano on Umnak Island in the eastern Aleutian Islands. The intensity of activity has been relatively steady for the past week and ash plumes reaching 10,000 - 20,000 feet have been observed in satellite images and by local observers. Periods of intense seismicity are not necessarily corresponding with episodes of significant ash production and this has complicated attempts to provide early warning about potential ash cloud hazards based on seismicity alone.

A crew of AVO scientists visited Umnak Island and viewed the eruption on August 2nd and 3rd. They report significant ash fall in the caldera and on the upper flanks of the caldera rim, lahars and lahar deltas forming in drainages from the southeast to northeast flanks, and a very vigorous eruption emitting continuous ash jets from three or more vents in the vicinity of Cone D in the northeast sector of the caldera.

Since the eruption began on July 12, seismicity, satellite data, and direct observations indicate that the eruption has remained robust but has waxed and waned in intensity. Nearly continuous volcanic tremor with intermittent bursts of very intense tremor have characterized the eruption thus far. Tremor may result from multiple processes including explosions and ash-gas emissions at the vents and the movement of magma beneath the surface. This may reflect the amount and rate of influx of water into the vent area resulting in pulses of vigorous steam and magmatic gas driven explosions lasting for minutes to almost an hour followed by periods of relative quiescence.

The current activity differs in character from the past three significant eruptions at Okmok in 1945, 1958, and 1997 all of which occurred at a cinder and spatter cone on the far southwestern portion of the caldera floor, Cone A. In general, each eruption was mildly to moderately explosive with most ash clouds produced rising less than 30,000 ft above sea level. Each eruption also produced a lava flow that traveled about 5 miles across the caldera floor. The current eruption has yet to produce a lava flow at the surface.

Prognosis and Hazards

Based on past eruptions at Okmok and our analysis of the current episode of activity, we would expect this event to continue for several weeks and possibly longer with ash emissions of varying explosivity. It is possible that the effusion of lava could ensue accompanied potentially by lava fountains, spatter accumulations, and possibly a lava flow that spreads across the caldera. It is also possible that explosivity could intensify again and sustained ash emssions resume.

As long as the volcano is actively erupting, intermittent trace to minor amounts of ash fall can be expected to continue downwind of the volcano including over marine areas adjacent to Umnak Island. The caldera and areas in the immediate vicinity of the volcano on Umnak Island should be avoided, particularly the caldera itself, the crater rim, and any drainages leading from the caldera rim. It is possible that further strong explosions can produce ballistics or larger particles of tephra that can impact the area around the caldera rim and surrounding area for several miles.

Also, if activity were to intensify, pyroclastic flows and surges--dangerous fast moving clouds of ash, larger particles, and hot gas-- could form and travel across the caldera floor as well as over the caldera rim and down slope. Mudflows or lahars may form as rain mixes with the ash fallout. These will likely be channeled down drainages from the caldera rim, especially to the northeast and to the south of the volcano, including Crater Creek, which drains to the Bering Sea northeast of the caldera. Depending on the evolution of activity within the caldera, Crater Creek may become dammed posing an extreme flooding risk in the Crater Creek Drainage.

AVO will continue to monitor the volcano closely. The operations room at AVO in Anchorage is staffed 24 hours per day. We will attempt to visit the volcano again as soon as conditions allow in order to better document and understand the activity and ongoing hazards. New data and observations may lead us to change our assessment. Any changes would be announced in a subsequent Volcanic Activity Notice. AVO appreciates the cooperation and assistance of mariners, pilots, residents of Unalaska and other Aleutian communities who are sending observations, ash samples, and photographs via our web site.

OTHER ALASKA VOLCANOES

Seismic activity is monitored in real time at 30 volcanoes in Alaska. Satellite images of all Alaskan volcanoes are analyzed daily for evidence of ash plumes and elevated surface temperatures. Some volcanoes may currently display anomalous behavior but are not considered to be at a dangerous level of unrest. Augustine, Iliamna, Redoubt, Wrangell, Gareloi, Great Sitkin, Makushin, Fisher, Shishaldin, Isanotski, Pavlof, Veniaminof, Ugashik-Peulik, Griggs, Snowy, Fourpeaked, Aniakchak, Tanaga, Kanaga, Akutan, Westdahl, Dutton, Ukinrek Maars, Martin, Mageik, Trident, Katmai, Novarupta, Spurr, and Korovin volcanoes are in color code GREEN and volcano alert level Normal. All are at or near normal levels of background seismicity. AVO did not detect ash plumes or significant elevated surface temperatures in the vicinity of any volcano.

The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.

VOLCANO ALERT LEVELS

NORMALVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

ADVISORYVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

WATCHVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway but poses limited hazards.

WARNINGHazardous eruption is imminent, underway, or suspected.

AVIATION COLOR CODES

GREENVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

YELLOWVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

ORANGEVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway with no or minor volcanic-ash emissions [ash-plume height specified, if possible].

REDEruption is imminent with significant emission of volcanic ash into the atmosphere likely OR eruption is underway or suspected with significant emission of volcanic ash into the atmosphere [ash-plume height specified, if possible].